Ancient rock offers peek at Earth

LOS ANGELES - Geochemists studying Arctic rocks say they have found evidence of ancient rock from the interior of the Earth that is nearly as old as the Earth itself.

Such material gives scientists an idea of what the mineral structure of the inner Earth used to look like billions of years ago and may force them to adjust their theories about the evolution of the plan- et's structure over the eons, said Matthew Jackson, a geochemist at Boston University and lead author of the paper.

The findings were published in Thursday's issue of the journal Nature.

Some scientists had said it was unlikely to find rock this old from the mantle, the solid part of the Earth's interior. Covered by the 7-to-25-mile thick crust, the mantle, which makes up four-fifths of the planet's volume, is hard to get to and study.

And even when samples are found, they are usually relatively young. That is because the mantle rock is constantly recycled, emerging at the surface as molten volcanic material and being replenished by parts of the Earth's crust that are pushed back down into it and melt.

Because of all the mixing, no uncontaminated examples of what the Earth's mantle looked like at the beginning of its formation had been found before now.

The rocks in the Nature study were spewed up to the surface of the Earth by an eruption about 60 million years ago, said Carnegie Institution of Washington geochemist Richard Carlson, one of the authors of the study.

They were obtained from a site on Baffin Island, in the Canadian Arctic. Using ratios of lead isotopes, the scientists were able to date the rocks to about 4.5 billion years ago, just shy of the Earth's age.